1. Field of the Invention
The present invention relates generally to Marx-type generators, and more particularly to an extremely compact Marx generator comprised of specially shaped components assembled into an integrated package. The invention particularly relates to (i) high-performance ceramic capacitors, (ii) high-performance miniature gas switches that can be repetitively operated, (iii) high-performance solid dielectric switches that can be operated with high-reliability, (iv) methods for coupling energy from stage-to-stage to overcome difficulties in switching successive stages, and (v) methods for manufacturing these high-performance components where electric field stresses are eliminated or sufficiently reduced in the so-called weak areas.
2. Description of Related Art
Conventional Marx-type high voltage generators use a voltage multiplier configuration in which many capacitors are charged in parallel to the input voltage, then the charged capacitors are switched into a series configuration to generate a high-voltage output about equal to the sum of the voltages across each of the capacitors. In principle, Marx generators are simple, and their capability is largely determined by the performance of the individual energy storage capacitors and switches used in each stage. But, a high-performance Marx generator that can store a rather large amount of energy in a relatively compact enclosure, produce a very high voltage in a relatively small enclosure and deliver a large current pulse with a relatively fast response time is quite difficult to build.
The performance that can be obtained from Marx generators ultimately depends on the physical layout and construction details as well as strengths of materials. Marx generators are often constructed with standard, commercially available components that are orderly arranged into compact assemblies. Because such components are made for general use, their packaging and terminal styles are not optimum for highly specialized applications that require very high voltages in very small spaces and very low inductance interconnections. The overall system performance of a Marx generator can be significantly improved by manufacturing special components that are designed to be part of an integrated package that uses dielectrics to their maximum advantage, minimizes inductances, and tightly controls stray capacitances.
Spark gaps are the most frequently used switches in repetitively operated Marx generators, and basically consist of two metal electrodes separated by a gap filled with pressurized gas, all in a dielectric housing. The spark-gap switch operates when the electric field in the gap exceeds the breakdown level of the dielectric gas. A spark gap component is generally much larger than the actual gap breakdown region within. This is a consequence of having to provide a suitable structural housing to support the electrodes and contain the pressurized gas. To make spark gaps hold off higher voltages, path lengths along the gas envelope surface or through the housing material must be made reasonably long, which increases the inductance of the switch and limits the performance of the Marx generator. Low-profile, low-inductance switches are needed for compact, high-performance Marx generators.
The physical layout of components is one element that directly affects the inductances and stray capacitances distributed within a Marx generator. In conventional Marx generators a delicate balance between the stray stage-to-housing capacitance and the interstage (switch) capacitance needs to be achieved for proper operation. Generally, the stray stage-to-housing capacitance must be large enough, and the switch capacitance small enough to achieve sufficient voltage increase across the next in-line switch to cause it to operate as a result of impulse coupling from the voltage collapse across the nearby switch that has already operated. Thus, each switch that operates triggers the next switch to operate in a progressive fashion that allows the Marx bank to erect properly.
Ordinarily, in conventional Marx generators the switch capacitance is low enough that stray stage-to-housing capacitance is adequate to couple enough voltage across adjacent switches to cause them to operate. However, in an ultra-compact Marx generator the extremely low-profile switch would have a switch capacitance so large that it becomes impractical to balance it out with a sufficient stray stage-to-housing capacitance. But this shortcoming can be overcome in the present invention by incorporating discrete feed-forward coupling capacitors across stages that effectively augments the stray capacitance and thus reduces the ill-effects of the increased low-profile switch capacitance.
The performance of any ultra-compact Marx generator can be no better than the strength of the materials being used, and the electrical or mechanical stresses they are exposed to. Materials selection is a highly developed art, less developed are ways to reduce stresses that allow the selected materials to do their job more effectively. However, the dielectric constant and strength of the capacitor material are very important because they co-determine the amount of energy that can be stored. The energy stored in capacitors typically increases in direct proportion to the dielectric constant and in proportion to the square of the dielectric strength. The dielectric strengths of the insulating materials used for the switches and housing are also quite important. In order to achieve the maximum overall energy density it is desirable to operate near the intrinsic breakdown levels of the materials, e.g., at the highest voltages possible.
The electric field stresses usually cause the most trouble in the critical regions where dissimilar materials meet, particularly at so-called triple-point regions where a metal electrode and two different dielectric materials are in direct proximity. At these locations electric field stresses can become severely enhanced, increasing as much as the ratio of the dielectric constants of the different materials, or more, depending on the shapes of the materials. In conventional Marx generators, the field enhancements are not of tremendous importance because spacings are usually sufficiently great, and appropriate safety factors are applied so that the weakest material is not overstressed; however, this generally results in a sacrifice of volumetric energy density. In the present invention, the adjoining metal and dielectric materials are specially shaped to reduce the electric fields at the triple-point regions to levels that are below the breakdown threshold of the weakest material at the triple-point regions. The electric fields in the weakest regions are preferably spread throughout the stronger regions, e.g., in the bulk of the dielectric materials themselves, rather than at their interfaces.
Another problematic area where electric field stresses can lead to failure is in the placement or attachment of electrodes next to solid dielectric materials such as the switch housings. Customarily, when the dielectric material of the switch housing is pressed or mechanically attached between electrodes, there exists small voids that fill with some other material, typically the gas or liquid used to insulate the entire Marx assembly. Usually this void filling material has a lower electrical strength than the solid dielectric material, and the electrical field in this highly stressed region may easily exceed the electrical strength of the void filing material. Moreover, for gas and many liquids having a lower dielectric constant than the solid dielectric, there can be a field concentration in the void region that enhances the likelihood of breakdown in this weaker material. By metalizing the surfaces of the solid dielectric materials in the present invention, wherever contact is to be made with metal electrodes, the electric fields are eliminated in the void regions, thus preventing electrical discharge (corona) activity that can lead to breakdown of the bulk dielectric material.
Solid-dielectric switches, where the dielectric material itself is what breaks down under an over-voltage condition, have never before been reported as being used in a Marx-type generator. However, for single-action operation, solid dielectric switches offer high reliability and extremely low inductance. Solid dielectrics usually can withstand, and then switch, substantially higher voltages than either gas or liquid dielectrics in a spark gap switch. Many plastics have intrinsic breakdown levels as high as 1 to 10 MV/cm, whereas a gas such as sulfur hexafluoride (SF.sub.6) has a breakdown level of approximately 75 kV/cm per atmosphere of pressure. The difficulty has always been that switches for a Marx generator must be able to operate reliably at a predetermined over-voltage level, and the standard deviation in operating level must be rather small, typically four to six percent, in order to be assured that all of the switches can be properly over-voltaged so the Marx bank erects reliably. By proper design and manufacturing of a controlled region of enhanced electrical stress, dielectric switches can be made reliable enough to be used in a Marx-type high-voltage generator.